The phenomenon of superconductivity--i.e., zero electrical resistance, exhibited by many metals at near absolute zero temperatures--has been under investigation for many years. Likewise, studies directed at the practical utilization of superconductivity have been underway for some time. However, as yet few uses have actually been made of the phenomenon, despite its obvious utility in such areas of technology as, for example, power transmission. The economic benefit of zero resistance and the resulting loss-free power transmission is thought to be so great as to outweigh the practical problems involved; for example, that of keeping the superconductive cable cold enough so as not to revert to the normally conductive state. Therefore, many attempts have been made to fabricate a useful superconducting power line. The present invention concerns itself with the realization of this goal and provides a superconducting cable which can carry a useful amount of either alternating or direct current (hereafter ac or dc).
The criteria for a workable power transmission line are essentially that it have low or zero power loss, be readily and economically manufactured, and be electrically stable so as not to be the weakest link in the system in which it is used. Bearing these criteria in mind, it is instructive to consider the application of superconductor technology to the transmission of large quantities of electrical power.
The superconductive property in all materials is defeated by exceeding any one of three limits: the critical temperature (T.sub.c), the critical current density (J.sub.c), and the critical magnetic field (H.sub.c), each measured at some given value of the other two. These parameters are to some degree interrelated as well; for example, the critical field decreases with an increase in current density. Thus far, the material which has the most favorable critical quantities and which is also practical to fabricate is niobium stannide (Nb.sub.3 Sn). While superconductors with higher critical values are known, as yet there has not been developed a commercially practicable method for their manufacture; therefore, this discussion will focus on Nb.sub.3 Sn.
A second group of design factors is the result of the current travel characteristics of ac and dc. While dc travels through a conductor as a whole (i.e., penetrates the bulk of the conductor), ac, of frequencies above roughly 30 Hz, travels in the outer layer of the conductor at a depth inversely proportional to the frequency--the so-called "skin effect." Hence, dc conductors are usually made in bulk shapes, but ac conductors of a given cross-sectional area are most useful in thin, sheet-like shapes. Furthermore, it is unavoidable that any dc current will have some fluctuations or impurities which behave as ac. Thus a monofilamentary dc conductor will carry the "pure" dc in its center and the ac fluctuations on its surface. A further point to be considered is that somewhere in every circuit, current must at any point in time be flowing in antiparallel directions. It is well known that ac in such circumstances will flow on the surfaces of the conductors which face each other; thus, in order to increase the total current-carrying ability of a given conductor, its design must be such that these facing surfaces are of maximum area.
In the design of superconductors, it is generally found that their performance can be greatly improved by positioning them in close proximity to a material of good electrical conductivity so that if they should for some reason "go normal" and return to the nonsuperconductive state, an alternative current path is provided, thus permitting the superconductor to once again lose all resistivity, and assume its proper function.
It is therefore the object of the invention to provide a stabilized superconductor suitable for carrying large currents of less-than-pure dc, or of ac.
It is a further object of the invention to provide a method whereby such a superconductor can be made economically and practically.
It is still another object of the invention to show how such a superconductor can be adapted for use in a low-loss power transmission cable.
Other objects and aspects of the invention will appear to those skilled in the art.